Make api clients with interceptors cloneable

[sprudel]

Make clients clonable:

• Remove the chained interceptor
• Make service account interceptor cloneable by managing token with interior mutablity

Note: this is built on top of #565

[buehler]

Hi @sprudel

While I like the idea of clonable interceptors, I don't understand why the chained interceptor should be removed. (1) it is a breaking change and (2) it was introduced to remove the need for generic and multiple impls for the client builder.

Since it is duplicate code, I'd love to have only one implementation for the client builder. Otherwise, we need to maintain several implementations and things can and will go wrong when some parts are updated.

[sprudel]

@buehler yes it is a breaking change, but when making the clients cloneable it created a lot of friction.
Compare #543 which created even more complexity by introducing a new trait. (But I didn't have a look if this could have been simplified).
Basically it felt to me not being a good abstraction, as it locks all properties such as Clone not only to the concrete interceptor but also to the impl of the ChainedInterceptor.

As far as I have understood it was only introduced to make the impl of the builder easy, or was there another reason?
For me a good builder should make it easy to construct a type, and not change the result, only because a builder is used.

I have iterated over the implementation and now found a way to have no code duplication by using generics in a smart way.
This also made it very easy to support all possible interceptors, which can be quite handy.

I pushed the modified version to this branch. What do you think of this version?

Still it is a fair argument to say such a breaking change is not worth it. Given #567 however, this will be needed either way.

[sprudel]

@buehler From my perspective this PR would be mergable. However, I'm not sure if using the generics to remove the chained interceptor still fulfills your requirements.

[buehler]

This is a pretty solid implementation. Thank you @sprudel. To be honest, I didn't know enough about rust types to make this kind of changes. It serves as a good example for me now ;-)

I like the implementation and how it is used.